Magnesium alloys are low-density metals and have high strength and high rigidity and are thus attract attention as lightweight structural materials. In particular, expanded materials are excellent in mechanical properties such as strength and toughness, and thus expected to be popularized in future. The properties of magnesium alloys are changed by changing the types and amounts of the metal elements added. In particular, alloys (for example, AZ91 on the basis of the ASTM standards) having high aluminum contents have high corrosion resistance and high strength and are in great demand as expanded materials. However, magnesium alloys have low plastic workability at room temperature because of the hexagonal close-packed crystal structure thereof, and thus press working of sheet materials are carried out at a high sheet temperature of 200° C. to 300° C. Therefore, the development of magnesium alloy sheets capable of stable working at as low a temperature as possible has been desired.
In producing a magnesium alloy sheet, various methods can be used. However, for example, die casting and thixomolding have difficulty in producing a thin alloy sheet and have the problem of producing many crystals in a magnesium alloy sheet produced by rolling an extruded material of a billet, increasing the crystal grain size, or roughening the surface of the sheet. In particular, in a magnesium alloy with a high Al content, crystals or segregation easily occurs in casting, and there is thus the problem of leaving crystals or segregated substances in the final alloy sheet even after a heat treatment step and a rolling step after casing, thereby causing a starting point of breakage during press working.
In a typical example of conventional known methods for producing a magnesium alloy sheet, a magnesium alloy blank is pre-heated to 300° C. or more and then rolled with a reduction roll at room temperature, the pre-heating and rolling being repeated.
Also, as a technique for producing a magnesium alloy sheet containing fine crystal grains for improving plastic workability, the method disclosed in Patent Document 1 is known. This method includes rolling a magnesium alloy blank at a surface temperature of 250° C. to 350° C. with a reduction roll at a surface temperature of 80° C. to 230° C.
Other known techniques for improving the plastic workability of magnesium alloy sheets are disclosed in Patent Documents 2 to 5.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-2378
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-27173
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-29871
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-294966
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2004-346351